1. Field of the Invention
The present invention relates to a ceramic capacitor, and more specifically it relates to a ceramic capacitor employed as a smoothing capacitor for a switching source.
2. Discussion of Background
Up to the present time, most smoothing capacitors for switching sources have been constituted of aluminum electrolytic capacitors. However, as the demand for both miniaturization and improved reliability have increased in the market, the need for a compact ceramic capacitor assuring a high degree of reliability has also increased.
Generally speaking, since a great deal of heat is generated in the vicinity of a source, substrates are normally constituted of an aluminum having a high heat discharge capacity. However, since the temperature in the vicinity of the source changes greatly when the source is turned on and off, a great deal of thermal stress occurs at a ceramic capacitor mounted on the aluminum substrate, which has a high coefficient of thermal expansion. This thermal stress causes cracking to occur at the ceramic capacitor, which, in turn, may induce problems such as shorting defects and arcing.
In order to prevent problems such as arcing, it is crucial that the thermal stress occurring at the ceramic capacitor be reduced. As a means for reducing the thermal stress, Japanese Examined Utility Model Publication No. 46258/1993, Japanese Unexamined Patent Publication No. 171911/1992, Japanese Unexamined Patent Publication No. 259205/1992 and the like disclose a structure achieved by soldering a metal plate onto a terminal electrode of the ceramic capacitor and mounting the metal plate onto the aluminum substrate to prevent the ceramic capacitor from being soldered directly onto the aluminum substrate.
Under normal circumstances, it is necessary to set the length of the leg portion of the metal plate extending from the terminal portion to be soldered onto the aluminum substrate to the portion where it is connected to the ceramic capacitor as large as possible in order to ensure that the thermal stress caused by the expansion and contraction of the aluminum substrate is absorbed to a sufficient degree. However, since products in the prior art adopt a structure in which the height of the ceramic capacitor is bound to increase if the legs of the metal plate are lengthened, the length of the leg of the metal plate must be restricted to ensure that it is less than the allowable height that is permitted on the substrate.
Because of this, the length of the legs of the metal plate cannot be set at a large value in the products in the prior art and, consequently, if the ceramic capacitor is continuously operated over an extended period of time in an environment where the temperature changes drastically (xe2x88x9255xc2x0 C. to 120xc2x0 C.), as in the vicinity of a source, cracks will occur near the ends of the ceramic capacitor, presenting a high risk of arcing. This gravely compromises the reliability of the ceramic capacitor and has been a obstacle to the wider use of ceramic capacitors.
In addition, the metal plate in the prior art is constituted of phosphor bronze, silver, copper, stainless steel, aluminum, nickel silver or the like. However, these metals all have a coefficient of average linear expansion that is markedly higher than the coefficient of average linear expansion of the ceramic dielectric material constituting the ceramic capacitor. Thus, if any of them is employed to constitute a component to be mounted in the vicinity of a source where the temperature changes greatly, a great deal of stress is applied to, in particular, the area where the metal plate is connected due to the difference between the coefficient of average linear expansion of the ceramic capacitor element and the coefficient of average linear expansion of the metal plate to result in cracking occurring near the ends of the ceramic capacitor, which may lead to problems such as continuity defects, arcing and the like.
Furthermore, ceramic capacitors achieving a large capacity by laminating a plurality of laminated ceramic capacitor elements, soldering metal plate terminals onto terminal electrodes of the individual laminated ceramic capacitor elements and electrically connecting in parallel the plurality of laminated ceramic capacitor elements have been proposed (e.g., Japanese Unexamined Patent Publication No. 188810/1992, Japanese Unexamined Patent Publication No. 17679/1996).
Normally, soldering paste containing solder particulates, rosin-type resin, an actuator and the like is employed to solder and secure metal plate terminals onto the terminal electrodes of laminated ceramic capacitor elements. The activator is constituted of a halogen compound containing chlorine and the like. The particle size of the solder particulate is set at approximately 1 xcexcm to 50 xcexcm. The rosin-type resin content is set within the range of 50 wt % to 55 wt %. The content of the activator which is constituted of a halogen compound containing chlorine and the like is set at approximately 1%. In addition, the distance formed between the individual capacitor elements when combining the laminated ceramic capacitor elements is maintained within a range of 10 xcexcm to 20 xcexcm.
However, when soldering the metal plate terminals onto the individual terminal electrodes of the laminated ceramic capacitor elements, the solder particles and the flux contained in the soldering paste enter the gaps between the laminated ceramic capacitor elements to result in buildup occurring due to the solder balls and the flux, presenting problems such as shorting defects between the terminals and deteriorated insulation.
It is an object of the present invention to provide a ceramic capacitor in which cracks, damage and the like can be prevented from occurring at the ceramic capacitor element with a high degree of reliability.
It is a further object of the present invention to provide a ceramic capacitor in which the thermal stress and the mechanical stress occurring at the ceramic capacitor element can be reduced.
It is a still further object of the present invention to provide a ceramic capacitor in which the length of the metal plate terminal extending from the terminal portion located toward the substrate to the ceramic capacitor element is increased without increasing its height.
It is a still further object of the present invention to provide a ceramic capacitor in which cracks, damage and the like can be prevented from occurring at the ceramic capacitor element with a high degree of reliability within a temperature range of xe2x88x9255xc2x0 C. to 125xc2x0 C.
It is a still further object of the present invention to provide a ceramic capacitor achieving an improvement in reliability by preventing solder particles and soldering flux from entering gaps between the ceramic capacitor elements.
In order to achieve the objects described above, the present invention discloses a structure of metal plate terminals, the material that should be selected to constitute the metal plate terminals, the correlation between the coefficient of linear expansion of the ceramic capacitor elements and the coefficient of linear expansion of the metal plate terminals and the soldering requirements to be fulfilled when soldering the ceramic capacitor elements and the metal plate terminals.
In regard to the structure of the metal plate terminals, the ceramic capacitor according to the present invention includes at least one ceramic capacitor element and at least a pair of metal plate terminals. Terminal electrodes are provided at the two diametrical side end surfaces of the ceramic capacitor element.
The metal plate terminals are each connected to one of the terminal electrodes at one end, and are each provided with a folded portion in the middle, with a terminal portion to be connected to an external conductor such as a substrate provided toward the other end of the folded portion.
In the metal plate terminals structured as described above, the folded portions increase the lengths which extend from the terminal portions to the ends that are connected to the terminal electrodes of the ceramic capacitor element. In addition, the folded portions achieve a spring-like effect. This ensures that the flexure and the thermal expansion of the substrate are absorbed with a high degree of reliability to reduce the mechanical stress and the thermal stress occurring at the ceramic capacitor element so that cracks can be prevented from occurring at the ceramic capacitor element. Consequently, even when the ceramic capacitor according to the present invention is employed as a smoothing capacitor for a switching source that is often mounted at an aluminum substrate, cracks can be prevented from occurring so that the risk of arcing can be eliminated.
In addition, by providing the folded portion at the metal plate terminal, the flexure and the thermal expansion of the substrate are absorbed to prevent mechanical stress and thermal stress from occurring at the ceramic capacitor element, and an increase in the height is prevented. Thus, the length of the metal plate terminal extending from the terminal portion located toward the substrate to the ceramic capacitor element mounting portions can be increased without increasing its height, to improve the absorbing effect with respect to flexure and the thermal expansion of the substrate so that the mechanical stress and the thermal stress occurring at the ceramic capacitor element can be reduced.
The metal plate terminals are each constituted of a metal material having a coefficient of average linear expansion xcex1 of 13xc3x9710xe2x88x926 or lower over the range of xe2x88x9255xc2x0 C. to 125xc2x0 C. It has been learned that by constituting the metal plate terminals with a metal material achieving such a coefficient of average linear expansion xcex1, cracks do not occur and the risk of arcing is eliminated even when it is used continuously over an extended period of time in an environment where the temperature changes drastically over the range of xe2x88x9255xc2x0 C. to 125xc2x0 C. Consequently, even when the ceramic capacitor according to the present invention is employed as a smoothing capacitor in a switching source that is turned on/off frequently and may experience a temperature fluctuation within the range of xe2x88x9255xc2x0 C. to 125xc2x0 C., a sufficient degree of reliability is assured. The coefficient of average linear expansion xcex1 as referred to in the present invention refers to the average value of coefficients of linear expansion measured at a plurality of different temperatures.
In regard to the coefficients of linear expansion of the ceramic capacitor element and the metal plate terminals, xcex11 less than xcex12 is satisfied with xcex11 representing the coefficient of average linear expansion of the ceramic capacitor element over a range of 25xc2x0 C. to xe2x88x9255xc2x0 C. and xcex12 representing the coefficient of average linear expansion of the ceramic capacitor element over a range of 25xc2x0 C. to 125xc2x0 C., and the coefficient of average linear expansion xcex2 of the metal plate terminals over the range of xe2x88x9255xc2x0 C. to 125xc2x0 C. satisfies xcex2 less than 1.3 xcex12 and xcex2 greater than 0.7 xcex11.
It has been confirmed that when the coefficients of average linear expansion xcex11, xcex12 and xcex2 satisfy the requirements presented above, cracks, damage and the like are prevented from occurring at the ceramic capacitor element with a high degree of reliability over the temperature range of xe2x88x9255xc2x0 C. to 125xc2x0 C.
When the main constituent of the dielectric is barium titanate, the coefficients of average linear expansion of the ceramic dielectric satisfies xcex11xe2x89xa67xc3x9710xe2x88x926 and xcex12xe2x89xa79xc3x9710xe2x88x926. When the main constituent of the ceramic dielectric is a lead type complex perovskite, xcex11xe2x89xa62xc3x9710xe2x88x926 and xcex12xe2x89xa73xc3x9710xe2x88x926 are satisfied.
Consequently, the coefficient of average linear expansion xcex2 of the metal plate terminals must be set by taking into consideration the different coefficients of average linear expansion xcex11 and xcex12 manifesting when the main constituent of the dielectric is barium titanate and when it is lead-type complex perovskite so that the requirements described earlier are satisfied in both cases.
The soldering requirements for soldering the ceramic capacitor element and the metal plate terminals are adopted when producing a combined ceramic capacitor constituted by combining a plurality of ceramic capacitor elements. The plurality of ceramic capacitor elements are each laminated while maintaining a distance of 20 xcexcm or less with their terminal electrodes soldered to the metal plate terminals. A soldering paste containing solder particles 90% or more of which achieve a particle size of 35 xcexcm to 55 xcexcm is used in the soldering process. In the combined ceramic capacitor achieved in this manner, the solder particles and the soldering flux do not enter the gaps between the ceramic capacitor elements. This contributes to an improvement in the reliability.